Sheet Molding Compound (SMC) and Bulk Molding Compound (BMC) are used frequently in the production of composite materials for use in the automotive, trucking agricultural equipment (“ag-equipment”), construction, electronics, aircraft and military industries. The molding compound contains a mixture of fibers, fillers, additives, and a liquid molding resin. These compounds are generally categorized as Bulk Molding Compound (BMC) or Sheet Molding Compound (SMC) depending on their compositions. SMC's usually contain higher amounts of reinforcing fiber than BMC. Manufacturing composite parts using these compounds is popular because of their ability to produce low cost parts with a wide range of properties. Additionally, they have the ability to overcome many of the difficulties encountered in composites manufacturing, such as shrinkage and surface defects, which are especially important in the automotive and ag-equipment areas, where surface finish is an important esthetic of the product.
The production of composite parts from Sheet Molding Compounds takes place in two separate steps, namely, compounding and molding. The compounding step involves the actual production of the “sheet” by blending the liquid molding resin, and optional fillers and additives into a paste, casting the paste onto a carrier film, and optionally depositing the chopped reinforcing fibers into the paste in order to reinforce the sheet, and then sandwiching the mixture with another layer of carrier film. These sheets are then allowed to age in a controlled environment for a predetermined amount of time. During this time, the thickening agents in the paste cause the viscosity of the paste to increase to a level suitable for easy handling and the molding step. The compound formulations, include but are not limited to, divalent cation complexation with carboxyl groups, chain telemerization via unreacted epoxide groups on the triglycerides reacting with di-acids, anhydrides, diamines or di-isocyanates. At this point, the “B-staged ” sheet becomes leathery in consistency and is easy to handle in high volume manufacturing operations. In the next manufacturing step, the sheet is cut into a suitable sized piece and placed into a compression mold. The mold is heated, causing some of the labile bonds of the B-Staged resin to weaken, enhancing the flow in the mold and activating chemical initiators within the SMC, thereby causing a cross-linking polymerization reaction. The cure reaction solidifies the molding compound and forms a thermoset composite part.
Liquid molding resins used in SMC are usually prepared by first synthesizing a low molecular weight polymer having the functional groups required for the cross-linking reaction. A reactive diluent is also usually added with the necessary chemical functionality to take part in the cross-linking reaction. If the cross-linking reaction is of the free radical addition type, the required functional group on the polymer is ethylenic unsaturation and the reactive diluent is also an ethylenically unsaturated compound such as, but not limited to, styrene, α-methyl styrene, divinyl benzene, and methyl methacrylate. The relative ratios of unsaturated groups on the polymer and the amount of the reactive diluent are important parameters that those knowledgeable in the field have learned to optimize.
Based on the type of functional group that takes part in the cross-linking reaction, particular types of catalysts and accelerators are also included in the resin which begin and facilitate the cross-linking reaction. If the cross-linking reaction is of the addition type, accelerators such as, but not limited to cobalt naphtenate, cobalt octoate, aromatic tertiary amines, and free radical initiators such as, but not limited to, methyl ethyl ketone peroxide, benzoyl peroxide, cumyl hydroperoxide, and t-butyl peroxybenzoate are added. The choice of initiators and accelerators depends on the reactivity of the polymer and the temperature and the time desired for the cure reaction. The choice of accelerators and initiators are well documented in the literature and are well known by those of ordinary skill in the art.
Commercially available liquid molding resins such as, but not limited to unsaturated polyesters, vinyl esters and epoxy resins are all synthesized using raw materials derived ultimately from petroleum. These include, but are not limited to di-acids and diols, among others, maleic anhydride, phthalic acid, isophthalic acid, aliphatic diols, bisphenol-A, acrylic and methacrylic acid, aliphatic and aromatic diamines, ethylene glycol, propylene glycol and other diols, all of which are petroleum derivatives. Replacing some, or all, of these petroleum derived raw materials with renewable plant-based raw materials is attractive, both economically and socially, as such raw materials are cheaper and their use contributes to global sustainability by not depleting scarce resources of petroleum.
The use of plant-based raw materials such as plant oils is further useful as such naturally occurring compounds are usually consumed readily by microorganisms. In fact, plant triglycerides are readily hydrolyzed in vivo by lipase secreting bacteria. Some chemically functionalized plant oils when reacted to form thermosets are biodegradable while others are not. This presents a novel method for adding and controlling biodegradability in the composite parts formed from these molding compounds. This aspect of these polymers is an additional advantage over polymers derived solely from petroleum based raw materials, very few of which are degradable by naturally occurring bacteria.
It is the intention of the present invention to use resins derived from these plant oils as components in a Sheet Molding Compound. By optionally blending these novel resins with fillers, additives, and reinforcing fiber, Sheet Molding Compounds can be produced which form materials with properties comparable to those produced from petroleum based Sheet Molding Compounds.
In the past, triglycerides and their fatty acid derivatives have been used only in the molding compound field as nothing more than additives. Much of their use has been as components of internal mold release agents. U.S. Pat. Nos. 4,408,000, 4,409,351, 5,576,409, 5,883,166, and 5,744,816 address the use of fatty acids and fatty acid amides as components in a mold releasing agent. The salts of fatty acids have also found use in mold releasing agents as presented in U.S. Pat. No. 4,144,305.
Triglycerides and their fatty acid derivatives have also found much use in coatings for SMC parts. U.S. Pat. No. 5,504,151 presents a surface quality enhancer composed of a polycapped oligomer adduct of a long chain fatty acid and a polyester polyol, which improves the efficiency of thermoplastic low profile additives. U.S. Pat. No. 4,367,192 mentions the use of epoxidized fatty acids to produce vinyl ester components for use in SMC coatings. Similarly, U.S. Pat. No. 4,245,059 presents a coating material for molding compounds where the coating is prepared by condensation of a polyol with a polybasic acid using a modifier such as linseed oil, soybean oil, as well as numerous other oils and fatty acid derivatives. U.S. Pat. No. 5,114,756 presents a conductive epoxypolyamide coating for SMC, formed by reaction of thermally polymerized fatty acids and a polyalkylene polyamine.
Fatty acids and their derivatives have also been used to modify or improve the properties of various Sheet Molding Compounds. U.S. Pat. No. 5,200,446 presents a plastic molding compound treated with an antistatic agent containing fatty acid esters or ethanolamides, mono- or di-glycerides or ethoxylated fatty amines. U.S. Pat. No. 4,101,475 covers a scratch and flame resistant Sheet Molding Compound in which one additive is the salt of fatty acids. Additionally, fatty acid amides have been used to aid in the thickening of Sheet Molding Compounds as presented in U.S. Pat. No. 3,879,318. In U.S. Pat. No. 4,444,921 a thermosettable composition suitable for preparing reinforced plastic articles is produced using a calcium carbonate filler coated with a fatty acid or fatty acid metal salt. Similarly, U.S. Pat. Nos. 4,210,572 and 4,210,571 present the use of fatty acids esters and hydroxy fatty acid esters as coupling agents for fillers in Sheet Molding Compounds. U.S. Pat. Nos. 5,268,400, and 5,100,935 present a Sheet Molding Compound containing various fatty acids as phase stabilizing agents in an unsaturated polyester Sheet Molding Compound.
Amongst all of the aforementioned work, the triglyceride and fatty acid derivatives were used only as additives to either aid in the processing of the Sheet Molding Compound or modify the properties of the end product. This invention differs in that the triglycerides and their fatty acid residues are used to form both the main polymer matrix and the thickening mechanism, roles conventionally taken by petroleum derived polyester, epoxy, and vinyl ester resins.